As currently in operation, the aerobic biological systems for the treatment of effluents with organic loads, called the activated sludge process, use two steps that perform different operations: there is a first, “Biological Reactor Step”, and a second “Gravitational Sedimentation of Solids Step”.
The first step in the activated sludge process is the biological reactor step, which occurs inside a biological reactor, which operates with aerobic bacteria, mainly containing means for the dissolution of the oxygen gas that may come from atmospheric air, or oxygen-containing gas in concentrations higher than the atmosphere, dissolved by equipment called aerators or oxygenators. The equipment used for dissolving gases also provides mixing energy that must be used to maintain the solids in suspension in the biological reactor. Among the solids are the microorganisms responsible for the absorption and processing of the organic load or nutrient load, resulting in its removal from the liquid medium.
In an activated sludge process system, the microorganisms used are aerobic, and the process requires the application of equipment that dissolves the oxygen gas in the liquid known as mixed liquor, which is the liquid to be treated plus the mass of microorganisms present in the biological reactor.
Additionally, in many chemical/fermentative processes where the dissolution of the air and/or oxygen is required, the overall rate of production of the process is nearly always limited by the transfer of oxygen to the liquid. This limitation is the object of many searches having an aim of improving such transfer rates.
The equipment used to promote the oxygenation are called aerators, blowers, diffusers or oxygenators. A variety of aeration systems are used in wastewater treatment processes.
Two basic methods of aerating wastewater are:
a. Dissolving air or oxygen in wastewater with submerged diffusers or other aeration devices; and
b. Mechanical agitation of wastewater that promotes the dissolution of atmospheric oxygen.
The aeration devices may be classified in a variety of ways, such as:
a. Suction devices: Consisting of a hydraulic propellant that pumps a flow of liquid mass which, by the venturi effect, sucks the air from the atmosphere and discharges the air/water mixture below the surface of the water. These devices have low oxygen transfer efficiency.
b. Static tubes: Consist of one or more tubes, mounted at the bottom of the biological reactor, which receive air from blowers that is dissolved in the liquid through holes in the tubes, aerating and promoting the mixture, however they have a low oxygen transfer.
c. Disc diffusers: Consist of rigid ceramic discs or flexible porous membranes mounted on air distribution tubes near the bottom of the biological reactor tank. They receive air from blowers to be dissolved in the liquid through the pores of the discs, which dispense bubbles of air into the liquid. The energy for mixing in this process is provided by the blown air transformed into bubbles released in the liquid, in an ascending movement.
d. Venturi aerators: Applied to aeration equipment, the venturi is responsible for sucking the air and promoting its mixture into the liquid in the form of small bubbles. A Venturi aerator is composed of a nozzle, followed by a constant-diameter conduit (throat) and then a gradually divergent cone. The throat area, being small, results in a high velocity of the liquid, followed by a corresponding decrease in static pressure which allows the use of the Venturi as a gas suction device. The use of a Venturi to suck gases uses as a criterion the obtaining of a pressure inferior to the atmospheric one. The existing holes in the throat wall provide suctioning of air or other gas due to the pressure differential.
e. Ejectors: These are devices that use the kinetic energy of a high velocity liquid jet to conduct, disperse and dissolve gases in liquids. The “jet aeration” aeration system, which is composed of hydraulic pumps for recirculation of mixed liquor from the biological reactor, falls into this category.
The second step in the activated sludge process is the gravitational sedimentation of solids step, which operates using decanters or gravitational settlers, which consist of compartments of low hydraulic turbulence that allow the microorganisms formed in the biological reactor to be separated via sedimentation at the bottom, and collected with or without the help of bottom scrapers which direct the sludge to a point of collection, where it are sucked up, the majority being recirculated to the biological reactor and part discarded for densification, dewatering and disposal.
The objective of recirculating the separated solids from the gravitational settler to the biological reactor is to increase the concentration of microorganisms in the biological reactor and thus increase the load removing capacity for the available biological reactor volume. In the conventional activated sludge process, the concentration of suspended solids in the biological reactor is around 2000 mg/L to 4000 mg/L. At these concentrations, the detention period in the gravitational settler process characteristically takes around 3 to 5 hours. In this compartment, the dissolved oxygen concentration is practically zero. This type of environment is called an anoxic environment. In such an environment, it is not recommended that an aerobic bacterium remain for more than 5 hours. To sediment sludge concentrations above 4000 mg/L, periods of decantation detention above 5 hours would be necessary, this period thus being the limiting factor of the concentration of microorganisms in conventional activated sludge projects.
The process of treatment of effluents by conventional activated sludge processes can therefore be defined as the hydraulic interconnection of two compartments, operating in two different steps, the first step, in a compartment called a biological reactor, aiming to develop a culture of microorganisms capable of absorbing and digesting the dissolved material in a mass of water, and in the second step, solid separation occurring within another compartment generally composed of gravitational settlers capable of separating the microorganisms that are in the solid phase. The liquid phase is the liquid contained in the aeration tank, free from the microorganism mass. While the solids are collected at the bottom of the settler, the liquid free of the microorganism mass is discarded through surface spillways.
The treated effluent is the decanted effluent, discharged by the spillways.
The mass of pollutants is absorbed by the microorganism and discarded as biological sludge. What is needed, therefore, is a system that makes it possible to increase the overall treatment capacity over conventional activated sludge processes.